1. Field of the Invention
This invention relates to chemical vapor deposition systems and, more particularly, to liquid delivery systems, and to heaters and vaporizers for use in such systems.
2. Description of the Related Art
Many materials are utilized in the form of thin films on substrates and are formed by vapor deposition techniques. Examples include refractory materials such as high temperature superconducting (HTSC) materials including YBa.sub.2 Cu.sub.3 O.sub.x, wherein x is from about 6 to 7.3, BiSrCaCuO, and TlBaCaCuO. Barium titanate, BaTiO.sub.3, and barium strontium titanate, Ba.sub.x Sr.sub.1-x TiO.sub.3, have been identified as ferroelectric and photonic materials with unique and potentially very useful properties in thin film applications of such materials. Ba.sub.x Sr.sub.1-x Nb.sub.2 O.sub.6 is a photonic material whose index of refraction changes as a function of electric field and also as a function of the intensity of light upon it. Lead zirconate titanate, PbZr.sub.1-x Ti.sub.x O.sub.3, is a ferroelectric material whose properties are very interesting. The Group II metal fluorides, BaF.sub.2, CaF.sub.2, and SrF.sub.2, are useful for scintillation detecting and coating of optical fibers. Refractory oxides such as Ta.sub.2 O.sub.5 are coming into expanded use in the microelectronics industry; Ta.sub.2 O.sub.5 is envisioned as a thin-film capacitor material whose use may enable higher density memory devices to be fabricated.
Many of the potential applications of these materials require their use in thin film, coating, or layer form. The films or layers may also be advantageously epitaxially related to the substrate upon which they are formed. Applications in which the refractory materials may need to be deposited in film or layer form include integrated circuits, switches, radiation detectors, thin film capacitors, holographic storage media, and various other microelectronic devices.
Chemical vapor deposition (CVD) is a particularly attractive method for forming these layers because it is readily scaled up to production runs and because the electronic industry has a wide experience and an established equipment base in the use of CVD technology which can be applied to new CVD processes. In general, the control of key variables such as stoichiometry and film thickness, and the coating of a wide variety of substrate geometries is possible with CVD. Forming the thin films by CVD will permit the integration of these materials into existing device production technologies. CVD also permits the formation of layers of the refractory materials that are epitaxially related to substrates having close crystal structures.
CVD requires that the element source reagents, i.e., the precursor compounds and complexes containing the elements or components of interest must be sufficiently volatile to permit gas phase transport into the chemical vapor deposition reactor. The elemental component source reagent must decompose in the CVD reactor to deposit only the desired element at the desired growth temperatures. Premature gas phase reactions leading to particulate formation must not occur, nor should the source reagent decompose in the lines before reaching the reactor deposition chamber. When compounds are desired to be deposited, obtaining optimal properties requires close control of stoichiometry which can be achieved if the reagent can be delivered into the reactor in a controllable fashion. In this respect the reagents must not be so chemically stable that they are non-reactive in the deposition chamber.
Desirable CVD reagents therefore are fairly reactive and volatile. Unfortunately, for many of the refractive materials described above, volatile reagents do not exist. Many potentially highly useful refractory materials have in common that one or more of their components are elements, i.e., the Group II metals barium, calcium, or strontium, or the early transition metals zirconium or hafnium, for which no or few volatile compounds well-suited for CVD are known. In many cases, the source reagents are solids whose sublimation temperature may be very close to the decomposition temperature, in which case the reagent may begin to decompose in the lines before reaching the reactor, and it therefore is very difficult to control the stoichiometry of the deposited films from such decomposition--susceptible reagents.
In other cases, the CVD reagents are liquids, but their delivery into the CVD reactor in the vapor phase has proven impractical because of problems of premature decomposition or stoichiometry control. Examples include the deposition of tantalum oxide from the liquid source tantalum ethoxide and the deposition of titanium nitride from bis(dialkylamide)titanium reagents.
When the film being deposited by CVD is a multicomponent substance rather than a pure element, such as barium titanate or the oxide superconductors, controlling the stoichiometry of the film is critical to obtaining the desired film properties. In the deposition of such materials, which may form films with a wide range of stoichiometries, the controlled delivery of known proportions of the source reagents into the CVD reactor chamber is essential.
While source reagent liquid delivery systems present distinct advantages over conventional techniques, there is often some fraction of the precursor compound that decomposes into very low volatility compounds that remain at the vaporization zone. This deficiency is an important issue in the operation of CVD processes that use thermally unstable solid source precursors which undergo significant decomposition at conditions needed for sublimation. Such decomposition can occur in all reagent delivery systems that involve a vaporization step, including flash vaporizer liquid delivery systems as well as more conventional reagent delivery systems that include bubblers and heated vessels operated without carrier gas.
Although well-behaved CVD precursors vaporized under "ideal" conditions will form no deposits or residue at the vaporization zone, deviations from this situation are common and can be divided into several categories:
1) CVD precursors are employed which are thermally unstable at the sublimation temperature. PA1 2) Reactive impurities in either the precursor or in the carrier gas decompose at the vaporizer temperatures. PA1 3) Spatial and temporal temperature variations occur in the vaporization zone, with temperatures in some regions being sufficient to bring about decomposition. PA1 1. A highly efficient structural configuration of the vaporization zone and process manifolds, and the provision of a process by-pass line and residue trap, which prevent undesired accumulation of non-volatile residue in the vaporization zone. PA1 2. The provision of manifolds in the process system and process by-pass conduits, having fluid flow conductance characteristics which minimize formation of non-volatile residue in the vaporization zone. PA1 3. The provision of means and method for removing the non-volatile residue that does collect in the vaporization zone.
Optimization of the conditions used in the vaporizer of reagent delivery systems can minimize the fraction of the delivered precursor that decomposes (and remains) at the vaporization zone, but virtually all solid and liquid precursors undergo some decomposition when they are heated for conversion to the gas phase. This fraction is negligibly small in "well-behaved" compounds. Use of precursors that tend to decompose near their vaporization temperature may be mandated by availability (i.e., where the selected precursor possesses the best properties of available choices) or by economics, where precursor cost is strongly dependent on the complexity of its synthesis.
Additionally, CVD precursors often contain impurities, and the presence of those impurities can cause undesirable thermally activated chemical reactions at the vaporization zone, also resulting in formation of involatile solids and liquids at that location. For example, a variety of CVD precursors (such as tantalum pentaethoxide) are water-sensitive and hydrolysis can occur at the heated vaporizer zone forming tantalum oxide particulates that may be incorporated into the growing tantalum oxide film with deleterious effects.
Despite the advantages of the liquid delivery approach (which include improved precision and accuracy for most liquid and solid CVD precursors and higher film growth rates), the foregoing deficiencies pose a serious impediment to widespread use of the vaporization liquid delivery technique for providing volatilized reagent to the CVD reactor.
A wide range of CVD processes currently under development utilize solid and liquid precursors. In many cases these chemicals are the only suitable choice in terms of gaseous transport of the element of interest, and this is especially characteristic of a number of metal elements such as Ba, Sr, La, etc. Ba, for example, is most easily transported in the vapor phase via incorporation in metalorganic compounds such as the beta-diketonates (e.g., Ba(thd).sub.2, Ba(thd).sub.2 -tetraglyme, etc.) With such types of compounds, the temperature needed to sublime the material is often &gt;200.degree. C., and the sublimation or evaporation process is accompanied by decomposition of some fraction of the compound, which results in formation of other metalorganic complexes with reduced volatility. These decomposition by-products tend to remain in the vaporization zone, where they collect and serve to progressively diminish the efficiency of the vaporization process. With continued accumulation of decomposition products, and concomitant reduction of efficiency, the rate of decomposition will increase. The accretion of non-vapor byproduct decomposition products thus limits the operating life of the vaporizer. Such products buildup will eventually render the vaporizer and delivery system incapable of even minimal operating efficiency, thereby requiring the removal of the byproduct accumulation from the vaporization and delivery unit by cleaning processes. These maintenance procedures may be very labor- and time-intensive, or may even involve complete replacement of the vaporizer/delivery unit. Accordingly, the operating (on-line) service life of the vaporization/delivery unit may be substantially foreshortened, relative to a corresponding vaporization/delivery unit which does not experience such accumulation of decomposition non-volatile byproducts.
As an example of the foregoing problems, the chemical vapor deposition of barium strontium titanate results in precursor decomposition in the vaporizer chamber, producing an involatile liquid byproduct. When the vaporizer cools, the involatile liquid becomes a brown glass containing all three elements (barium, strontium and titanium). Some flows toward a trap, but some also remains on the vaporization element, typically a frit structure as described more fully in prior copending application No. 08/484,025 filed Jun. 7, 1995 in the names of Peter S. Kirlin, et al. The glassy residue remaining on the frit element decreases its conductance and progressively deteriorates the vaporization efficiency of the liquid delivery system.
It is therefore desirable for high efficiency operation of the liquid delivery system that the formation of non-volatile residue in the vaporization zone be minimized, that the tendency of the vaporizer to collect the non-volatile residue be minimized, and that suitable techniques, preferably in-situ techniques (interiorly applicable in the vaporization zone), be readily performable to remove the residue that does collect in the vaporization zone.
It is also desirable for the vaporizer component of the liquid delivery system to perform consistently, without perturbation or process variation, after the glassy residue has been removed in each maintenance cycle, relative to its performance preceding such maintenance. The vaporizer is also desirably easy to install and to maintain. Additionally, the liquid delivery system should be designed to readily detect leaks, to accommodate simple user operating support and maintenance procedures, and to be of a compact character, with a minimum of zones to heat in operation of the system.
Since the vaporizer must be heated to a temperature in excess of 200.degree. C., with a high spatial uniformity of .+-.5.degree. C., it is also desirable for the vaporizer heater to maintain the vaporization chamber and associated conduits within the same temperature window.
Accordingly, it is an object of the present invention to provide an improved delivery system for introduction of CVD source reagent precursors to CVD reactors.
It is another object of the invention to provide an improved liquid source reagent vaporization apparatus for a CVD process system which suppresses the formation of source reagent non-volatile decomposition byproducts and minimizes their passage to the CVD reactor.
It is a further object of the invention to provide an improved liquid delivery system comprising in-situ source reagent non-volatile decomposition byproduct removal and disposition means.
It is a further object of the invention to provide an improved vaporizer that is readily maintained, repaired and replaced, and that is compact and highly efficient in use. Other objects and advantages of the present invention will be more fully apparent from the ensuing disclosure and appended claims.